Trends in particulate nutrient concentrations and seston stoichiometry of the Laurentian Great Lakes.

Offshore particulate matter is typically planktonic in origin; changes in concentrations and elemental ratios can reflect surrounding ecological conditions and aid in assessing ecosystem health. The EPA's Great Lakes National Program Office has collected particulate nutrient data in both spring and summer from all five of the Laurentian Great Lakes since the late 1990s. Here, we present a cross-lake comparison of temporal trends in particulate phosphorus (PP), particulate nitrogen (PN), particulate organic carbon (POC), total suspended solids (TSS), and seston stoichiometry (N:P) from 1997 - 2019. Significant spring trends over the full time series include increased PP in Lake Superior, increased PN in central and eastern Lake Erie, decreased PP, PN, POC and TSS in Lake Huron, and decreased PP and TSS in southern Lake Michigan. The only significant trends in summer data were declines in epilimnion PP and TSS in Lake Michigan. After over a decade of general declines, PN concentrations across most lakes started increasing in 2012, with a significant break point in that year for lakes Superior, Huron, Michigan, and Ontario. Particulate N:P has changed in recent years, indicating possible nutrient deficiency in the lower food web, and the recent changes may hint at declining seston nutritional quality. We also compare particulate nutrient temporal patterns to the timing of significant ecological changes in the Great Lakes, including the expansion of dreissenid mussel populations, declines in the spring phytoplankton bloom, and changes to the zooplankton community. [ABSTRACT FROM AUTHOR]